1. Field of the Invention
This invention is directed to a filtered feedthrough assembly having at least one lead wire. In particular, this invention directs itself to a feedthrough assembly utilizing at least one chip capacitor coupled between a lead wire and a metallic ferrule. Still further, this invention directs itself to a filtered feedthrough for use in implantable medical devices wherein the capacitor is affixed in position by both conductive and non-conductive materials. Still further, this invention directs itself to a filtered feedthrough wherein any one of a plurality of capacitor elements can be individually replaced if found defective during testing.
2. Prior Art
Filtered feedthrough devices are well known in the art. The best prior art known to the Applicant include U.S. Pat. Nos. 3,329,911; 3,443,251; 3,617,830; 3,879,691; 4,152,540; 4,314,213; 4,424,551; 4,642,589; 4,673,900; 4,675,629; 4,682,129; 4,698,605; 4,700,155; 4,700,440; 4,772,225; 4,791,391; 4,804,332; 4,819,130; 4,853,824; 4,872,085; 4,887,185; 4,984,129; 5,032,949; 5,150,086; 5,153,540; 5,206,786; 5,213,522; 5,287,076; 5,333,095; 5,406,444; and the following publications: "Cellular Phones May Affect Use of Pacemakers", The Wall Street Journal, Friday, Apr. 28, 1995, pp. B1 and B3; "EMI Filtering in Medical Implantables", Medical Device and Diagnostic Industry, September 1994; "Do European GSM Mobile Cellular Phones Pose a Potential Risk to Pacemaker Patients?", Pace, Vol. 18, June 1995, pp. 1218-1224; and, "Ceramic EMI Filters--A Review", Ceramic Bulletin, Vol. 67, No. 4, 1988, pp. 737-746.
Filtered feedthrough devices have been employed in implantable devices, as disclosed in U.S. Pat. No. 4,152,540 and the publication entitled "EMI Filtering in Medical Implantables". Such filtered feedthrough devices are known to employ discoidal capacitors in single lead feedthrough devices and discoidal capacitor arrays in multi-lead assemblies. However, while discoidal capacitor arrays can be very space efficient, a single faulty capacitive element in such discoidal arrays caused the loss of an entire device, as such capacitive elements were not individually repairable and were very difficult to remove once installed. Further, the feedthrough devices incorporating such capacitive elements had to be specially manufactured to provide a cavity into which the capacitor or capacitor array was to be located, either by creating a recess within the ceramic hermetic sealing element, or extending the metallic ferrule beyond the hermetic seal in order to form such a cavity. Thus, even where a single discoidal capacitor is employed, such becomes extremely difficult to replace after having been bonded in position within such a cavity. Whereas in the instant invention, individual chip capacitors are utilized and positioned either on an end surface of the hermetic seal insulator or on a separate substrate which is subsequently married to the feedthrough. By this arrangement, filtering can be added to a conventional feedthrough device without the requirement for manufacturing special parts, thereby improving the efficiency of the manufacturing operation and allowing replacement of any chip capacitor with relative ease.
In still other prior art devices, such as that disclosed in U.S. Pat. No. 3,617,830, filtered feedthrough devices utilizing chip capacitors are disclosed. Such prior art devices disclose chip-type filter capacitors positioned between a pair of conductive rings, with the space between capacitors and the rings being encapsulated with an epoxy filler material. In addition to such materials not providing a high reliability hermetic seal, nor providing a biocompatible structure, such structures do not provide for accommodating capacitors of different sizes, as each chip capacitor is located within a cavity defined by the space between the conductive rings. Therefore, a different diameter outer ring is required for an application requiring larger capacitors, and in any one application all of the capacitors must be of the same length irrespective of their capacitive value. With the chip capacitors being disposed within a cavity, the difficulty in replacing any one capacitor which proves to be defective is almost as difficult as in the case where discoidal capacitors are utilized.
In systems such as that disclosed in U.S. Pat. No. 4.152,540, and other prior art systems such as that disclosed in U.S. Pat. Nos. 4,424,551 and 5,333,095, filtered feedthrough devices employing discoidal type capacitors are electrically coupled to the respective lead wires and ferrules utilizing conductive adhesive compositions. However, such systems provide no means by which defective devices can be easily replaced. Further, the system disclosed by U.S. Pat. No. 5,333,095 provides no means for applying a moisture resistant coating to the discoidal capacitive element which has a diameter substantially larger than that of the feedthrough device, making such impractical for use in most modern implantable systems wherein space is at a premium and where the feedthrough devices are manufactured by other than the implantable device manufacturer.
Outside the feedthrough art, it is known that chip capacitors may be employed in combination with ferrite blocks in connectors to form electromagnetic interference filters and electrically connected between the connector pin and the connector housing, as disclosed in U.S. Pat. No. 5,213,522. While a capacitive-inductive filter is formed by this arrangement, the individual capacitive elements are not easily replaced subsequent to their installation, as each capacitor is disposed within a cavity formed in a ferrite block, with the ferrite block being disposed within a cavity formed by the connector housing.
It has long been known that medical implantable devices must operate in an environment which is subjected to electromagnetic interference (EMI). The electrical leads which extend from such implantable devices act as antennas which receive and conduct electromagnetic energy into the electronics of the implanted device. Since the circuits of such medical implantable devices are very sensitive, and reliability is so important, as a fault may be life threatening, the medical implantable devices have incorporated filter circuits therein to suppress EMI. In some cases, feedthrough devices employing discoidal type capacitors have been employed in an attempt to filter out the electromagnetic interfering signals before they reach the electronic circuitry of the implantable device. The ability to filter the interference before it reaches the electronics has become more important recently, with the discovery that the electromagnetic interference generated by the new digital cellular telephones and other electronic devices is not sufficiently suppressed by prior art electromagnetic interference filters of some current medical implantable devices. The high frequency emissions from digital cellular telephones may be re-radiating within the medical implantable device, bypassing and thereby rendering the "on-board" filters ineffective. It is therefore critical that the filtering take place as close to the source of the emissions as possible, such as at the entrance to the housing of the implantable device. As a result of this ever increasing problem, the use of filtered feedthroughs will be required to effectively suppress EMI, and therefore it will be important to efficiently manufacture such in order to help contain the costs of the implantable medical devices. Thus, it will be important to be able to avoid the scrapping of whole assemblies when one component thereof is found to be defective.
Further, for medical devices such as implantable defibrillators, the high voltage output through the feedthrough devices adds another complexity to incorporating a filter capacitor therewith. A capacitor employed in such a device must be physically larger in order to withstand the higher voltage which will be impressed thereon, but the space limitations of the feedthrough used in such defibrillators are not conducive to accommodating large-sized capacitors. However, utilizing the filtered feedthrough of the present invention, such larger capacitors can be installed without requiring an increase in the physical size of the feedthrough structure. Additionally, by the arrangement of the filtered feedthrough of the present invention, any capacitive element found to be defective can be replaced, thereby avoiding the necessity for scrapping a complete assembly, or even a capacitive subassembly. Still further, to suppress high frequency EMI, each lead of a multi-lead feedthrough may be required to be individually tuned. Thus, each lead may require a capacitor having a different capacitance, voltage rating, or the like, which can be installed in the present invention.